This invention deals generally with electric lamp and discharge devices and more specifically with the support structure for the filament of an electron tube.
A typical power tube filament operates at a temperature of approximately 2200 degree s centigrade, and this can lead to severe structural problems. Not only is it necessary to support such filaments against structural movement when they are at such high temperatures, but it must be kept in mind that the filaments are not always at that temperature. Since the tubes must be turned on and off for various reasons, the filament will actually vary in temperature from near room temperature up to and including its operating temperature. Moreover, operational considerations require that the tubes must turn on rather quickly, thus causing the temperature of a filament to change at a very rapid rate.
This extreme temperature and dramatic temperature change places severe thermal stress, not only on the filament itself, but on the entire support structure of the filament. This occurs because the support structure generally is subjected to filament temperatures at one of its extremities and, therefore, temperatures throughout the support structure, even remote from the filament, are also very hot.
In a typical high power tube, such as a 90 KW continuous wave magnetron, in which the high power exaggerates the problems, this thermal stress can cause fracture of the typical filament support structure. In that particular type tube, it has been standard practice to use a helically wound tungsten filament. This configuration is supported at its lower end by a molybdenum weld ring which is essentially an inverted cup comprising a planar portion to which is attached a cylindrical side portion. The top of the inverted cup, the planar portion, has a central hole with a lip around the circumference of the hole, and the bottom of the helical filament is welded to this lip. Once assembled, the weld ring looks very much like a skirt attached at the bottom of the helical filament.
The top end of the filament is welded to a disc-like fixture which has a cylindrical protruding lip to which the filament is attached. This top disc is attached to and supported by a conductive rod which passes through the centers of the helical filament, the lower weld ring, and the rest of the filament support structure in order to both support the remote upper end of the filament and to act as an electrical connector for that end.
The lower filament weld ring also acts as the electrical connector at the lower end of the filament to which it is attached. The molybdenum weld ring is itself attached to, supported by, and receives the electrical power for the filament through an iron filament support cylinder around which the lower lip of the cylindrical skirt of the filament weld ring fits.
It is the filament weld ring which is most affected by the thermal stress to which the entire assembly is subjected. The relatively short filament weld ring has the extremely hot temperature of the filament attached to the central lip of its planar portion and the iron filament support cylinder attached to the bottom lip of its cylindrical portion, thereby subjecting the filament support cylinder to heat conducted through the weld ring. It is not uncommon, especially in tubes with high power ratings, for the braze between the filament weld ring and the filament support cylinder to crack because of the differential thermal expansion between the filament weld ring and the filament support cylinder to which it is attached.
This problem is aggravated by the materials required to be used for the various parts. The filament weld ring is typically constructed of molybdenum so that it may be welded to the tungsten filament and also withstand the high temperature, while the filament support cylinder, which is also attached to the filament weld ring, is typically constructed of iron because of the required magnetic properties. Since iron has a dramatic increase in its coefficient of thermal expansion when it rises above 900 degrees C., the increased temperature within the higher power tubes is at least part of the problem for the cracking of the bond between the parts. The iron filament support cylinder expands when heated and contracts when cooled much faster than the molybdenum filament weld ring does, and the braze at their junction tends to crack under the stress of the differential expansion and contraction.
Until now, the only means by which this problem has been alleviated has been to provide the cylindrical side portion of the filament weld ring with slots to relieve the mechanical stress caused by the expanding support cylinder. Such slots permit the fingers formed between them to deflect as heating causes the support cylinder to expand, and prevents the outer cylinder of the weld ring from resisting the expansion.
However, for the higher power tubes now being built, the stress relief afforded by the slotted construction has not been completely effective. The braze between the filament weld ring and the filament support cylinder continues to crack, and the addition of more slots in the skirt of the weld ring is limited by the requirement of the weld ring to conduct large filament currents, thus requiring a large cross sectional area for the conductive path. Additional slots would reduce this cross sectional area.